The present invention regards the area of orthopaedic surgery, and in particular a method of and devices for ensuring that prosthesis components are inserted correctly upon implantation of artificial hip joints into humans. In particular, the invention is directed at a method of and devices for ensuring the correct mutual positioning of the prosthesis components.
An artificial hip joint has two main components; a prosthesis stem and a cup. One end of the prosthesis stem is provided either with a spherical ball head or a prosthesis neck on which can be placed a ball head, where the ball head is designed for a close, sliding fit in a spherical recess in the cup. Together, the prosthesis stem with the ball head and the cup will act as a ball joint to replace the natural ball joint.
The other end of the prosthesis stem comprises an elongated part designed to be attached to the hollow femoral canal in the patient's femur.
The cup is designed to be attached to the cavity on the patient's pelvis. The hemispherical shaped recess in the cup is linked with an exterior surface designed to be attached to the pelvis, via a side face. The exterior surface may have various shapes, all according to the method of attachment to the pelvis and other choices made by the supplier. Several of the cups that are in use are shaped as an approximate hemisphere, where the outer hemispherical surface is designed to be cemented to the pelvis. The side face that connects the recess and the exterior surface may be flat or possibly inwardly sloping towards the recess, which is preferably approximately centred in the side face.
The prosthesis stem and the cup may be fixed to the femur and the pelvis respectively by using cement, or through a cement-free force fit. The invention may be used with both fixation techniques.
When replacing a worn out hip with a prosthesis, the head of the femur is replaced. This is done by cutting the neck of the femur and hollowing out the top of the femoral canal in order to make room for the elongated prosthesis stem that is either cemented into the hole or force fitted.
The cavity on the pelvis is milled out to receive the cup, which is then fixed either by means of cement or a force fit.
If the ball head is detachable, this is placed on the prosthesis stem before the ball head is placed in the cup, the joint is assembled by lifting the patient's leg up to a natural position and inserting the ball head in the recess in the cup, whereupon the incision is closed.
Such a prosthesis should give the patient a mobility that approximates that which is provided by the natural joint. Moreover, it is important that a “natural” movement of the joint does not cause the patient to get in a situation where the leg ends up in positions where the neck of the prosthesis rides on the edge of the cup. This may cause luxation of the joint, where the head of the prosthesis jumps out of the cup. This happens through simple leverage. Luxation occurs in the case of between 3 and 10% of all patients who have had a femoral prosthesis put in. If this happens, the patient must be anaesthetised before the joint is put back into place. Some patients must have a new operation. The risk of luxation is much greater in patients whose prosthesis components are assembled so as to have an incorrect mutual positioning, than in those where the mutual positioning of the components is correct.
The inventor has previously shown that an optimum mutual relationship between the prosthesis stem and the cup under experimental conditions (not published) results in a reduced risk of luxation because the patient can go through the everyday natural range of motion (ROM) without the parts of the prosthesis ending up in such mutual positioning so as to entail a risk of luxation.
The inventor has previously shown (not published) that the most adequate ROM is achieved by assembling both prosthesis components in a manner so as to give them a forward angle of about 15 degrees relative to the frontal plane of the body, while the cup forms an angle of 45 degrees with the horizontal plane. In medical terminology, forward angling is termed anteversion.
The inventor has also previously shown (not published) that even though the optimum is to have each of the components angled forwards at 15 degrees, the result is nearly as good if the sum of the forward angling of the two components is 30 degrees. Thus a prosthesis joint where the cup is angled forwards at 5 degrees and the prosthesis stem is angled forwards at 25 degrees will result in a ROM for the patient that is nearly as adequate as if both components were angled forward at 15 degrees, the sum of the forward angling being 30 degrees is both cases
Today there are no means available to ensure that the surgeon installs the prosthesis components with this correct mutual relationship. With today's methods therefore, this is done as judged by the eye. This judgement may be sufficient, especially for experienced surgeons who carry out a considerable number of this type of operation each year. It is estimated that 80% of all implantations of artificial hip joints are carried out by surgeons who do less than 20 of these every year. This number is not sufficient to get enough practice. It is therefore desirable to have a method and means that ensure a correct mutual positioning of the main parts of the prosthesis in order to reduce the possibility of errors, and thereby also reduce the risk of luxation with the resulting pain for the patient, and a possible second operation.